NuTeV - A High Energy Physics Experiment

A Tour for the Non-Physicist

Welcome to NuTeV!
My name is Len, and I will
be your tourguide today. I am a high school science teacher who has
joined the NuTeV collaboration, and I am giving this tour because I
think, to paraphrase an old saw, physics is too much fun to be left to
the physicists.

I am going to begin by showing you our detector and explaining what
its component parts do. Since my work here has been on various aspects
of the detector I think of it as the heart of the experiment, but the
folks working on our beamline would quickly point out that without
them we would have nothing to detect, and of course the test beam
people are always telling us that calibration is the most important
part of any experiment! If we have time, I will try to tell you a bit
about all these aspects of this experiment.

Standing here on this little loading dock the top of the
detector is
almost at eye level, and the neutrino beam is coming in from our
right. The first thing you probably notice is that this baby is BIG!
The front face is ten feet square, and the overall length from front
face to the blue cart is about 120 feet. The next thing to notice is
that there are three distinct chunks, the target-calorimeter, the
toroid magnet (muon spectrometer), and the blue cart way over to your
left.

Let's first take a closer look at the target-calorimeter. You can see
that this is a large "Dagwood Sandwich" (for those of you old enough
to know who Dagwood is), made up of three kinds of slices. The first
and heaviest type of slice is a two inch thick ten foot square steel
plate. Altogether there are 690 tons of these, and they comprise the
target -- that is, they provide some protons and neutrons for our
neutrinos to interact with. It is worth remembering here that this
seemingly solid steel is in fact mostly empty space. You probably know
that Ernest Rutherford, back in 1910, shot some alpha particles
through a gold foil, and discovered from the way these particles
scattered that an atom has a nucleus that contains almost all of the
atom's mass, but takes up only a tiny fraction of the apparent size of
the atom. The rest of the space is where the electrons orbit, and the
fact that electrons repel each other keeps the atom from getting
squashed. In iron, if the individual nucleii were the size of golf
balls, two "nearest neighbor" nucleii would be over a mile apart! It
is no wonder then that most of the neutrinos entering our target go
right on through -- that is why this detector has to be so large.

The next sort of slice in our sandwich is a scintillation counter,
again ten feet square, and an inch thick. There are 84 of them in
our target/calorimeter. These are in fact plastic
boxes, filled with scintillation oil, and with a photomultiplier tube
looking in at each corner. Any charged particle that passes through
the oil will cause it to scintillate; electrons are disturbed by the
passage of the charged particle, and as they settle down, they give
off photons of light. The photons are detected by the phototubes and
an
electrical signal is produced. The signal strength is proportional to the
energy deposited by the particle, and thus this acts as a calorimeter,
or energy measuring device. A second important feature of
scintillation counters is that they respond very quickly, allowing us
to pinpoint the time of a particular event accurately.

The third sort of slice is a drift chamber, used to determine the
position of a charged particle passing through; we have 42 of them in
the target, and can thus determine the track of a particle, by
linking up positions from chamber to chamber. To get a handle on how a
drift chamber senses position, think about shooting a BB gun through a
harp! Someone with perfect pitch could tell you which string the BB
hit, just by listening. Our drift chambers are a bit more complicated
than that, but I will try to make their operation clear for you. First
of all, each chamber is more like two harps, with the wires at right
angles, so we can get both an X and a Y position for the
chamber. Secondly there are only 24 wire assemblies in each plane, so
we certainly don't expect the particle to hit the wire in order to be
detected. What really happens is that as a charged particle passes
through the gas in the chamber it ionizes some gas molecules - that is
it knocks some electrons loose. The wire is held at a high positive voltage
and thus attracts the electrons, and the electrons arriving at the
wire produce a signal. The time it takes the electron to drift to the
wire is proportional to the distance from the wire the particle passed
through. Remember we already know from the scintillation counter WHEN
the particle passed, so by measuring the time delay until the drift
chamber signal occurs we find out WHERE it passed. Those of you who
are still paying attention are going to ask "how do you know which
side of the wire the electrons came from?" The chambers in our target
actually have two sense wires very close together in each cell, to
answer this question for us. Additionally, there are some field
shaping elements in each cell, but you don't really have to know about
them at this point.

That then is the target/calorimeter. In this close-up picture, the
scintillation counters have phototubes on the upper corner, the iron
plates are red primer colored, and the drift chambers are the silvery
slices without phototubes. The various cables bring power to and data
from the detector elements.You can see that what is measured in the
target
is the energy deposited by any charged particle either stopping or passing
through, and the track or path a particle follows. I will talk more
about this when we get around to talking about exactly what kind of
interactions we are looking for at NuTeV. First though, let's look at
the other big part of this detector - the muon spectrometer.

The muon spectrometer is in fact a large toroidal magnet - a few big
iron doughnuts with coils of current-carrying copper wire to produce a
magnetic field. There are scintillation counters and drift chambers
interspersed along the axis of the toroid to track particles passing
through. As a charged particle passes through a magnetic field, it
feels a force and its path is therefore bent. The amount of bend
indicates the momentum of the particle, with low momentum particles
being bent more than high momentum particles. (It is really the path
that is bent, not the particle, but physicists talk about this so
often that they have adopted this shorter way of saying things!) Our
toroid is used to measure the momentum of muons passing through the
detector, thus we call it a muon spectrometer. The blue cart sitting
way over to the left holds more drift chambers and is really a part of
the spectrometer. It gives us a longer lever arm for determining how
much a particular muon was bent.

One last thing to notice out here is all those blue cables leading
from the detector back into our trigger room; everything we "see" in
a modern high energy physics experiment is seen electronically, and
computers are absolutely indispensible for collecting, sorting, and
analyzing the data that we get. We will go into the trigger room
shortly, but first I should talk a bit about the particular kinds of
events we are looking for on this experiment.

OOPS! Do you hear that sort of high-pitched whine? That is our "Droege
Trip Alarm" telling me one of the drift chamber power supplies has
tripped because it was drawing too much current. It's my
responsibility to go take care of it, so I'll have to suspend the tour
for now, but I will continue at this URL soon. In the meantime you can
take heart in knowing that it is not just in your high school physics
lab that these sort of things happen - it is a part of The Joy of Physics!